The invention relates, in general, to spectrophotometers and, more particularly, to spectrophotometer optical sections. Spectrophotometers can be divided into two broad classes of systems known as forward optics and reversed optics systems. In forward optics systems, the optical beam of the spectrophotometer passes through a monochromator before passing through the sample cell so that only a single frequency of light passes through the sample cell. The sample spectrum must therefore be measured in a serial manner by sweeping the light frequency through the spectral range while measuring the absorbance of the sample. In reversed optics systems, the beam passes through the sample cell before passing into a monochromator or spectrograph which disperses the beam. An advantage of the reversed optics systems is that the entire spectrum of the light passes through the sample cell so that the transmittance spectrum can be measured by a parallel detector system such as a spectrograph.
A typical reversed optics system known in the prior art is shown in FIG. 1. The optical beam originates from a light source such as a lamp 11. Light from a filament 12 of source 11 is focussed by a lens 13 onto a source slit 15 in an optical barrier 14. A lens 16 focusses the light which passes through slit 15 onto a sample cell 17. A lens 18 focusses the part of the beam which passes through the sample cell onto a detector slit 110 in an optical barrier 19. The series of lenses 13, 16, and 18 cooperate to produce an image of filament 12 onto detector slit 110. The light which passes through slit 110 strikes a diffraction element such as focussing grating 111 and the light in the diffraction pattern is measured by a detector such as photodiode array 112.
Although reversed optics systems provide the speed advantage of allowing parallel detection of the transmitted spectrum, such systems are subject to producing spurious absorption. This problem arises because the detector slit in a reversed optics system is typically much smaller than that used in a forward optics system. To maximize beam throughput (i.e. the efficiency of transferring light from the source to the detector) the beam should be selected to have a cross section comparable to the cross section of the detector. Thus, reversed optics systems present the problem of striking a small slit with a narrow beam so that small deviations in beam direction due, for example, to sample cell misalignment or variation in the index of refraction of the sample, will produce variations in light intensity on the detector. Such variations will appear in the resulting spectrum as spurious absorption of the sample.
The detector slit is made small in reversed optics systems for a pair of reasons. First, a small detector slit reduces the amount of stray light entering the spectrograph and therefore improves sensitivity. Second, parallel detection of the spectrum is typically achieved by use of an array of detectors, such as a photodiode array. Reduction of the size of the detector slit reduces the size of the image at the detector so that a larger number of detectors can be employed in an array of given dimensions. Thus, reduction of detector slit size improves the resolution of a reversed optics system. Referring to FIG. 4A, both throughput and aberration increase with increase in the solid angle subtended by focussing grating 411 at either detector slit 410 or at photodiode array 412. Hence, a compromise between throughput and aberration requires that the size of the detector slit be approximately equal to the size of a photodiode in array 412. Therefore, resolution cannot be improved by reducing the magnification of focussing grating 411, but instead, for a given size of array 412, requires reduction in the width of slit 410.
In the prior art there are known a number of schemes to avoid such spurious absorption. In one method, the optical elements are precisely manufactured and precisely aligned so that variation of the sample cell alignment cannot occur, but such a system cannot correct for beam deflection due to variation of the index of refraction of the sample. This method requires the sample cell to be rigidly mounted and precisely aligned and does not correct for other sources of beam deflection. Such a system is thus not amenable to easy or rapid exchange of sample cells. Interchange of sample cells is important for the different requirements of various chemical and physical measurement situations. A second scheme, illustrated in FIG. 2, employs a detector slit which is wide enough to pass the image irrespective of any expected beam deflections. Such a wide slit has the disadvantage of allowing a large amount of stray light into the spectrograph which reduces resolution and sensitivity. A third scheme known in the prior art, illustrated in FIG. 3, produces at the detector slit an image which is very much wider than the slit thereby providing uniform intensity near the slit, so that small deflections of the beam will not vary the light intensity on the slit. This uniformity is typically achieved by focussing one-and-a-half coils of filament 12 onto the detector slit as illustrated in FIG. 3. This method, however, has a low throughput of beam to the spectrograph. Such low throughput is unsuitable for parallel detector systems because the reduced area of the individual detector as compared to the area of detectors employed in prior art forward optics system has increased the need for high throughput. In addition, a large area exposure of the sample is necessary in this method, making such a system susceptable to problems with sample photodegradion.